In a mobile communication system, a base station and a mobile station can both measure quality of received radio signal, so that the radio signal received from an apparatus of a wireless connection destination can be appropriately reproduced. Then, the base station and the mobile station can determine a transmission power control (TPC) value used as an index for determining transmission power of a radio signal according to the measured quality, and transmit the TPC value to the apparatus at the wireless connection destination.
The base station and the mobile station measure, for example, a signal-to-interference ratio (SIR) to determine the quality of the received radio signal.
For example, in cases where a code division multiplexing access (CDMA) is adopted as a multiplex system, a base station detects pilot symbols by applying a despreading operation to the received radio signal. The base station performs channel estimation using the detected pilot symbols, and then performs Rake combination. Then, after performing the Rake combination, the base station measures a receiving power of a pilot symbol period in an uplink-dedicated physical control channel (UL-DPCCH) and a power of interference signals. The base station acquires an SIR estimation value by dividing the received power of the Pilot symbol period by the power of the interference signals. Then, the base station compares the SIR estimation value with an SIR target value and determines a TPC bit value based on the comparison result. The base station transmits an uplink TPC bit value to the mobile station over a downlink-dedicated physical control channel (DL-DPCCH). Inner loop power control is realized by these operations.
Conventionally, a base station and mobile station automatically control gain (AGC) of the radio signal so that the radio signal received through an antenna can be reproduced. Therefore, the power value of the received signal and the power value of the interference signal, which are used to calculate the SIR estimation value in the base station, are relative to a total power value of the radio signal received by the base station. Thus, a change in the total power value of the radio signal received by the base station causes the power value of the received signal and the power value of the interference signals to change. For example, when the total power value of the radio signal received by the base station increases, each of the power values of the received signal from each mobile station and the power value of the interference signals relatively decrease.
The power value of the interference signal is measured by averaging signal distribution values of the pilot symbols in respective time slots over a plurality of slots. In order to estimate the power value of the interference signals correctly, the length of the period used for the averaging is set, for example, as a length corresponding to a plurality of frames, such as ten frames. However, a case may occur where the total received power in the base station is rapidly changed due to a change in the number of the mobile station connected with the base station in a same cell. In such case where the rapid change of the total received power occurs, the power value of the interference signals may be unable to follow the rapid change of the total received power since the power value of the interference signals is an average value of the values measured over a certain period as described above. Therefore, an error between the SIR estimation value and the actual value of SIR may become large, and as a result, there is a possibility that the mobile communication system may not perform normal transmission power control.
Techniques for adjusting the length of the period for calculating the estimate value of the interference signals according to the change of the power value of the received signal have been developed (refer to Japanese Laid-open Patent Publication No. 2001-197017 and Japanese Laid-open Patent Publication No. 2005-167710, for example).
For example, in such known techniques, while the apparatus for measuring the SIR detects desired wave power by the received signal, the apparatus detects interference wave power from the received signal. The apparatus applies an averaging process of earlier and later interference wave power detection values over a long period slot and an averaging process thereof over a short period slot. Furthermore, the apparatus obtains a difference of the two average values, thereby detecting an amount of the change of the interference wave power. The apparatus selects the average value over the short period in cases where the detected amount of the change of the interference wave power is large, and selects the average value over the long period in cases where the detected amount of the change of the interference wave power is small. Then, the apparatus obtains a ratio of the selected average value and the desired wave power.
According to another example of a known technique, a calculator for instantaneous interference wave power calculates a spread value of reverse modulation symbols in a predetermined period, and outputs the value as an instantaneous interference wave power value. Moreover, a calculator for the number of averaging interference wave power calculates a number of averaging interference wave power, which is short if the amount of interference wave power change calculated by a calculator for the amount of interference wave power change is more than a threshold, and calculates a number of averaging interference wave power, which is long if it is less than a threshold. An interference wave power averaging device performs an averaging process to the number of the instantaneous interference wave power values corresponding to the number of averaging interference wave power, and outputs the averaged value as an interference wave power value. Moreover, the calculator for the number of averaging interference wave power calculates the number of averaging interference wave power according to an estimate value of propagation channel environment. The estimate value of propagation channel environment is calculated based on any of the amount of phasing change, the amount of other cell interference, the number of multipasses and the number of other cell paths, or combinations thereof.